(1) Field of the Invention
The invention relates to the general field of emission devices and arrays, more particularly to the design of large displays.
(2) Description of the Prior Art
Cold cathode, or field emission, devices are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
The advent of semiconductor integrated circuit technology made possible the development and mass production of arrays of cold cathode emitters of this type. In most cases, cold cathode field emission displays comprise an array of very small conical emitters, each of which is connected to a source of negative voltage via a cathode conductor line. Another set of conductive lines (called gate lines) is located a short distance above the cathode lines at an angle (usually 90.degree.) to them, intersecting with them at the locations of the conical emitters or microtips, and connected to a source of positive voltage. Both the cathode and the gate line that relate to a particular microtip must be activated before there will be sufficient voltage to cause cold cathode emission.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and strike a cathodo-luminescent panel that is located a short distance from the gate lines. In general, a significant number of microtips serve together as a single pixel in a monochrome display or as sub-pixels in a color display. Note that, even though the local electric field in the immediate vicinity of a microtip is in excess of 10 million volts/cm., the externally applied voltage is only of the order of 100 volts.
The above described components are normally housed in a flat, vacuum tight, structure consisting of a front anode plate, a rear plate that bears the microtips as well as the power (cathode and gate) lines, separated from one another by side supports, located at their edges. Spacers, located between pixels, are also often provided for added strength, but may be omitted in small units.
The structure is evacuated to a high degreee of vacuum, including post seal gettering, through vacuum piping. The latter are attached, either at the side supports or to a front or rear plate, additional space on the surface being made available to accommodate them. It is then operated by means of external leads that are connected, through glass-metal seals, to the power lines. In general, a standard power supply, located near the array, is used for driving the power lines through the same leads that were used for operating.
In practice, it is not cost effective to manufacture field emission displays that exceed a certain size. In general, field emission displays do not measure more than about 30 inches by 30 inches because of the difficulty of manufacturing and handling glass sheets of uniform thickness that are larger than this. Unlike integrated circuits where many perfect chips may be obtained from a single wafer even if yield is relatively low, each field emission display must be perfect.
Thus, to date, field emission displays have tended to be limited to small area applications (usually having a diagonal measurement of less than 40 inches). This is unfortunate because they represent a highly energy-efficient technology and are able to provide the high level of brightness required by outdoor displays such as score boards or timetables. Additionally, field emission displays are well suited to video applications, should these be needed. The energy efficiency of field emission devices compares with that of the currently used large area technologies as follows:
______________________________________ TECHNOLOGY WATTS/LUMEN ______________________________________ HV-VFD* 15 LED** 25 FED 30 ______________________________________ *high voltage vacuum fluorescent displays **light emitting diodes
Note that for very large displays (large in size but having fewer pixels) the cost per pixel of the initial hardware favors LED devices, but in the size range around 300K pixels (corresponding to a 200 inch diagonal) FED displays, constructed according to the teachings of the present invention, are more cost effective aside from their improved energy efficiency.
In principle, it should be possible to combine many small field emission units into a large display by mounting them side by side. In practice, this is not done because, except for pixel pitches greater than about 5 mm., the separation between modules is found to substantially exceed the separation between pixels. As a result, the assemblage is not seamless and can be seen to be a composite. In practice, the pixel pitch of a display must be designed to be more than twice the separation distance between modules so if, for example, the separation is 2 mm., then the pixel pitch will need to be over 4 mm. Thus, the separation between modules represents the final limitation to the resolution of this type of display.
Additionally, multiple power supplies are needed to drive the multiple modules. Their associated wiring requires leaving room for wiring channels, further limiting the degree of compactness that can be achieved.
In FIG. 1, we illustrate how two individual modules, 1 and 2, are combined into a single unit using currently available methods. Each unit consists of a front plate 3, a rear plate 4, and edge plates 7. Optional spacers 8 are also seen. A set of edge plates, lying in a plane parallel to that of the figure, is not shown. A set of power lines (also not shown) is located on the inside surface of rear plates 4 and contact is made to them through electrode 6 which passes underneath an edge plate 7 and emerges as lead 5. This allows two contacts 6 to be combined into a single lead 5.
The present invention is directed towards improved module designs that can be seamlessly linked together to form a large area display. In the course of searching for prior art, the following references were found to be of interest. Te Velde (U.S. Pat. No. 5,238,435 August 1993) shows a liquid crystal display having very small cell thickness. Komano (U.S. Pat. No. 5,375,005 December 1994) discloses a liquid crystal display device for effectively supporting liquid crystal plate and illuminating device. Shannon et al. (U.S. Pat. No. 5,234,541 August 1993) shows a method of fabricating a MIM type device that is carried on a support together with an array of electrodes and address conductors for an active matrix display. Zimmerman (U.S. Pat. No. 5,603,649 February 1997) describes a self-aligned structure that allows FED devices to be fabricated in any desired size.